1. Field of the Invention
The invention relates generally to methodology and apparatus for dewatering sludge and for concentrating liquid suspensions, the invention particularly relating to accomplishment of such functions by subjection of the sludge to freezing temperatures while rotating the suspension.
2. Description of the Prior Art
Sludge dewatering presents a major waste water treatment problem at all waste water treatment facilities regardless of the age and treatment modalities of the system. On average, more than half of the total cost of waste water treatment can be attributed to sludge handling and disposal. An energy efficient dewatering technology could substantially reduce waste water treatment cost. Recognizing this, previous efforts have been directed to development of mechanical dewatering apparatus, particularly for sludge dewatering and also for other water purification steps. It has long been known that the freezing of an aqueous solution containing impurities can be utilized on a small scale to separate water from the solution. Such prior methodology requires freezing at a controlled rate to form pure ice crystals which are then separated from the impurity-containing residual solution, thereby removing water from the original solution as well as concentrating the impurities within the residual solution. This prior freeze technology is somewhat less than practical due to the large amounts of energy required for freezing and, in some cases, even for thawing of the ice. The art has experienced a long felt need for methodology and apparatus capable of taking advantage of the natural freezing conditions which exist seasonally in most areas of the United States. Such methodology and apparatus would also need to be amenable to use with the more economical methods of artificial freezing.
Examples of prior art systems directed to the problem noted above can be seen in a number of United States patents including U.S. Pat. No. 3,385,074 to Aronson wherein a system for extracting a relatively pure solvent from a solution is disclosed. A solvent such as water is flash frozen by Aronson to form crystals, thereby increasing the concentration of solute in the non-frozen portion which remains. Aronson includes in his process the centrifugation of the resulting material at high speed to separate the concentrated solute from adhered ice crystals.
Dreyer, in U.S. Pat. No. 3,400,548, discloses a process for the separation of crystals of 2,6-dimethylnaphthalene from solutions containing a plurality of dimethylnaphthalenes. A rotating drum with a surface cooled below the crystallization point of 2,6-dimethylnaphthalene continuously moves the surface of the drum away from contact with the liquid composition at a rate sufficient to form a solid layer of crystals on its surface.
U.S. Pat. No. 3,001,293 to Wendt discloses a process and apparatus for dewatering sludges, the process utilizing centrifugation steps which prevent losses of suspended materials. Cleveland, in U.S. Pat. No. 2,149,252, discloses a centrifugal process and apparatus for dewatering material which is adaptable for dewatering activated or other sewage sludges. In Cleveland, water is forced contracentrifugally towards the axis of the centrifuge and then through a filter for discharge.
Hultsch, in U.S. Pat. No. 4,101,421, and Wilson et al, in U.S. Pat. No. 3,814,246, disclose methods for centrifugally removing liquid from a mixture through use of a centrifuge equipped with a rotating screen or filter. Many other references exist in the art to a freeze-thaw treatment of materials including materials such as sewage sludges, such freeze-thaw treatments often including a centrifugation step to which the frozen sludge is subjected. Such patents include the following U.S. Pat. Nos.:
3,681,931, Hadzeriga PA1 3,803,861, Ganiaris PA1 3,817,048, Burley PA1 3,820,349, Styron et al PA1 3,885,399, Campbell
Netherlands patent 73 12616 also discloses a centrifugal separation of sludge-like materials.
The prior art, including those patents noted above, do not provide methodology and apparatus that is capable of dewatering sludges in an economical fashion by freezing of said sludges while rotating the sludges within a cylindrical container, a process particularly taught according to the present invention as efficiently dewatering sludges and/or concentrating impurities in aqueous materials. The present technology also allows for advantageous use of naturally existing low temperatures during certain seasons of the year and also for taking advantage of economical artificial methods of producing temperatures capable of freezing aqueous sludges and similar materials. The present invention is thus seen to provide advantages over the prior art and to contribute to cost reduction of sludge handling in waste water treatment facilities.